1. Field of the Invention
The invention pertains to a method and an apparatus for the improvement in the quality of well-head natural gas and in the prevention of pipeline corrosion in natural gas transport through the removal carbon dioxide (CO2) naturally occurring in the gas.
2. Brief Description of the Problem and the Prior Art
The term natural gas refers to mixtures of inert and light hydrocarbon components as well as non-hydrocarbon components which are recovered from natural gas wells or from gas coproduced in the production of oil. The removal of CO2 as well as other gaseous impurities has been the subject of much work in the past. The overwhelming majority of this work is directed toward the separation of CO2 from natural gas as opposed to its conversion to a different molecular species. These separations are typically performed through absorption or adsorption methodologies or alternatively through chemical scrubbing using techniques such as chemical chelation. The disadvantage of these current techniques lies mainly in their cumbersome characteristics which require replenishment of consumable chemical absorbents, adsorbents, or complexing agents. This renders these techniques less than optimal for remote application at a well-head.
The presence of CO2 and water is well known to play a role in the corrosion of pipelines. Corrosion of natural gas pipelines raises costs by both necessitating the replacement of pipelines and by the concomitant production loss during the resulting downtime. CO2 in the presence of water is in equilibrium with carbonic acid, bicarbonate, and carbonate ions. The pH-modifying nature of these species renders them corrosive to the pipelines used in natural gas transport. The chemistry is represented below:
CO2+H2O⇄H2CO3
H2CO3⇄HCO3xe2x88x92+H+
HCO3xe2x88x92⇄CO3xe2x88x922+H+
The problem can be alleviated by the removal of water, CO2, or both. One way to remove water is to lower the water dew point below the pipeline temperature. Temperature control through the expected distances of a natural gas pipeline is likely to be logistically difficult and cost prohibitive. Water may also be remove by various adsorption and absorption techniques that can be applied to CO2. Again, the regeneration or replacement of consumable chemical adsorbents and absorbents is a disadvantage of this method. Traditional natural gas purification by CO2 removal alone has also been based on adsorption and absorption.
An object of the present invention is a chemical process that is a self contained with respect to the replenishment of chemical reactants and heat, and which converts carbon dioxide in well-head natural gas to methane prior to pipeline shipment to non-remote locations. The primary goal of the present invention is the prevention of corrosion to transmission pipelines of natural gas through the removal of carbon dioxide, while a further object of the present invention is an environmentally friendly mechanism to remove CO2 from natural gas without venting to the atmosphere. Additionally, the present invention is useful for the improvement in quality of a natural gas effluent from a well-head prior to transmission to other locations.
U.S. Pat. No. 5,938,819 describes the bulk separation of carbon dioxide from methane using natural clinoptilolite. This is a zeolite-type chemical species whose mode of action is inclusion complexation, a non-covalent form of binding which is generally reversible under mild conditions. Purification of the gas stream is achieved through selective inclusion complexation. These types of adsorption systems are characterized by the need to regenerate the adsorbent species. In the ""819 patent, this is achieved through a technique commonly known as pressure swing adsorption (PSA). The system requires a supply of dry air for regeneration of the CO2 adsorbate. The requirement that reactants and/or adsorbents must be replenished or regenerated with an external supply of dry air detracts from the ease of remote application of the process at the well-head.
U.S. Pat. No. 5,411,721 describes the removal of CO2 from natural gas through a combination system utilizing membrane permeability selective techniques as well as pressure swing adsorption. Separation systems based on membrane permeability typically require high pressures, while those that rely on pressure swing adsorption are relatively inefficient at higher pressures. The ""721 patent combines the two techniques in such a way that the permeate feedstream is fed to the PSA system after passing the membrane. This nicely takes advantage of the pressure drop across the membrane in a a two step system is employed which minimizes pressure constraints and results in high purity, but has the disadvantage of a relatively high degree of complexity. The more complex a system of purification is, the less amenable it is to remote application at the well-head. The systems amenable to remote applications are ideally less cumbersome.
U.S. Pat. No. 5,089,034 uses multiple stage temperature swing adsorption (TSA) to purify natural gas by stepwise removal of H2O and CO2. By first removing water, the gas stream can later be treated at a lower temperature in the second adsorption zone for more efficient and less expensive carbon dioxide removal. This is based upon different chemistry and is of greater complexity than the present invention which makes the system less desirable for remote applications.
Still other systems separate impurities from natural gas streams through techniques of countercurrent chromatography. This technique is essentially a liquid-gas extraction. U.S. Pat. No. 5,660,603 uses an aqueous liquid, ideally seawater, as the liquid extraction medium. By using seawater at selected temperatures and pressures, hydrates of CO2 or other light hydrocarbons are formed. The liquid extraction medium is regenerated by the release of the complexed impurity gases by variations in temperature or pressure. It differs from the present invention in having the obvious disadvantages of requiring externally supplied heat and pressure. This results in a complex operation again not optimally suited for remote application at the well-head. Rather it is better suited for downstream use at a gas processing facility prior to distribution to gas customers.
Other and further chemistries are employed to selectively remove gaseous impurities from natural gas streams. U.S. Pat. No. 4,871,468 describes a method of removing hydrogen sulfide and carbon dioxide using a mixture of a polyvalent metal chelate in a carbon dioxide selective absorbent solvent at varying values of pH. Replenishment of solvents and reagents necessary to control pH are disadvantages here. The overall process involves reasonably complex solution chemistry it is better suited for a processing facility than a remote location such as well-head.
Numerous other patents exist that are variations on the same themes. Unlike the present invention, they variously employ complexation chemistry or selective adsorption and/or absorption as the separatory step. U.S. Pat. No. 4,741,745 employs a liquid-gas extraction technique where a liquid adsorbent is judiciously chosen. U.S. Pat. No. 4,409,102 combines a liquid-gas adsorption in a countercurrent extraction mode with a chemical scrubbing step.
All of this prior art differs fundamentally from the present invention in the chemistries employed. None of them are based on the removal of CO2 through its conversion to methane, and all are characterized by the need to regularly replenish reagents or require an external supply of heat and/or pressure.
In the preferred embodiment, a method and processor is used to purify a natural gas stream by removal of carbon dioxide through a methanation reaction which converts the carbon dioxide to methane by using molecular hydrogen obtained by chemical means from a portion of the natural gas stream to be purified. The system therefore consists of two reaction chemistries.
In one embodiment, a side stream of the main natural gas stream is diverted to a combustion device to generate heat that may be used to drive one or both of the chemical reactions. Such heat is also used to internally regenerate catalyst. The transfer of heat from the combustion device to one or both of the reactions may be regulated by ordinary means.
In the preferred embodiment, the methanation reaction is catalytic. In this embodiment, the catalyst interface may be configured in any number of conventional ways. Typically, this takes the form of columns or beds. Non-conventional forms may be used as well. One example of the methanation reaction is that commonly used in analytical gas chromatographic applications for the detection of carbon dioxide. This involves the use of a nickel-based catalyst and a temperature of 380xc2x0 C.
The hydrogen separator reaction chamber consists of a hydrocarbon reformation reactor in the preferred embodiment. Preferred chemistries are those commonly used in hydrocarbon reformation chemistry, particularly fuel cell technology. These typically use nickel, platinum, or palladium based catalyst.
The process involves redirection of side streams of raw natural gas to the hydrogen separator reaction chamber where the molecular hydrogen is extracted for subsequent use in combination with the main natural gas stream in the methanation reaction chamber. Upon methanation of the carbon dioxide in the raw natural gas stream, a purified stream emerges which is less concentrated in carbon dioxide than is the raw stream.
Further objects of the present invention utilize analogous chemistries substituted for the methanation reaction chemistry and/or for the hydrogen separator reaction. They involve the extraction of hydrogen from the main gas stream and the subsequent use of the extracted hydrogen to convert carbon dioxide to methane.
Other and further objects, features, and advantages would be apparent and eventually more readily understood upon a reading of the specification and by reference to the accompanying drawings forming a part thereof, and the examples given therein of the presently preferred embodiments of the invention.